1. Field of the Invention
The present invention relates to an image processing method and system capable of achieving a good representation of gray and other colors.
2. Description of the Related Art
Conventional ink-jet recording systems have a gray axis adjustment capability or black/color adjustment capability that allows for adjustment of an image's gray tone. Some users like cool gray, while others prefer warm gray, and yet other users prefer neutral gray. In view of the above, it is known to present a set of images having different gray tones so that a user can select a desirable tone.
FIG. 12 shows an example of a conventional test pattern for use in black and color adjustment. The test pattern includes a background and various patches. The background is printed using only black ink so that the tone of the black ink can be evaluated from the background. The patches are printed using color inks so that they have colors obtained by a mixture of three colors: cyan, magenta, and yellow. In FIG. 12, 99 patches 01 to 99 are arranged in an array such that the proportion of cyan, magenta and yellow is gradually varied from one patch to another thereby varying the color tone of the patches. From the 99 patches, the user then selects a patch having a color most similar to the tone of the background color printed with black ink, and the gray axis is adjusted depending on which patch is selected. Hewlett-Packard Co. employs this method, for example, in its printer CP1160.
FIG. 13 shows an example of a conventional test pattern image used to adjust the gray axis. In this example, each patch of the test pattern is a monotone image of a woman, and the 99 patches of FIG. 12 are arranged such that the color tone of the 99 patches is gradually varied from one patch to another by varying the ratios of color inks of cyan, magenta, and yellow. When the user selects a patch with the most desirable color tone, the gray axis is adjusted based on the selected patch.
However, in the conventional adjustment methods, the tone of low saturation colors is difficult to determine, although the relationship between gray and black or a set of color inks can be determined and the gray axis can be adjusted so as to obtain a desirable gray tone. This is because the adjustment is performed in terms of the gray axis and black.
Low saturation colors refer to colors having low saturation close to the gray axis. Specific examples include human skin color, the color of a stone or concrete building, the color the textual pattern of a mountain, ground color and the color of animal hair. Note that colors cited herein as examples are known as memory colors. These colors are strongly influenced by the setting of the gray axis. For example, if the gray axis is somewhat shifted toward blue, the skin color region close to the gray axis is also shifted toward blue along with the gray axis. As a result, the skin color becomes more bluish and less yellowish. Memory color refers to a color that is in memory of observers or a color that is incorrectly believed to be so although the actual color is different. If a color represented in a photographic picture is different from a corresponding memory color, a user will evaluate the picture as being significantly low in picture quality, even when the printed color is very similar to the real color.
When data is R=G=B, this data is gray-scale data. In many recent color ink-jet printers, in order to achieve fine granularity, gray colors with low lightness are obtained in the form of composite black using C, M, and Y inks, even when K (black) ink is available. In this case, the numbers of printed dots of C, M, and Y are controlled by the printer driver (implemented by software) or the like so that the resultant color seems like an achromatic color with coordinates of a=b=0 in CIELab color space.
To deal with color tone variations, Japanese Unexamined Patent Application Publication No. 2001-16475 discloses control of the color tone of image data such that colors fall within a range of 90° in the ab plane of CIELab color space. Moreover, Japanese Unexamined Patent Application Publication No. 2003-209702 has also disclosed control of the gray tone toward a negative direction along the b axis in the ab plane of CIELab color space such that high-fidelity gray is represented on a display or such that memory colors are represented in a desirable fashion. Users seek to achieve high-fidelity representation on a monitor or to achieve a good representation for memory colors by controlling the gray tone toward the negative direction along the b axis in the ab plane of CIELab. In silver halide monochrome photography, in contrast, most b-values are positive or located close to the origin. Thus, the target gray tone is not necessarily equal for all given images. This makes it difficult to achieve a desirable color tone in a printed image, depending on given image data.
As for colors other than gray, for example, blue, it is desirable, in some cases, to achieve a good representation of a memory color for the sky or the sea. But, in some cases, a good representation of a memory color is not needed, as in the case of an image of clothes.